The characterization or measurement of the flow of a multi-phase fluid presents numerous difficulties. A multi-phase fluid is a fluid having more than one phase (liquid or gas), such as a fluid having two or more liquid phases or a combination of a gas phase with one or more liquid phases. Attempts have been made to overcome these difficulties given the recognized need in industrial applications for the accurate characterization or measurement of the flow of such multi-phase fluids. For example, the oil and gas industry requires accurate measurement of the production of multi-phase fluids, comprising oil, hydrocarbon gases, water and/or other associated fluids, from underground reservoirs through wells in order that the production from each well can be assessed, managed and allocated in a reliable and consistent manner. In addition to the oil and gas industry, similar needs exist in other industries such as the chemical industry.
Generally, measurement of the flow of a multi-phase fluid presents difficulties due to the wide variety of flow regimes which are possible, general flow instability and the likelihood of a slip between the phases of the fluid due to segregation. For example, in a production well, the multi-phase fluid is likely comprised of oil, water and hydrocarbon gas. A slip may occur between the oil and water resulting in the production of separate slugs or plugs of oil and water. Meanwhile, the gas may take the form of small bubbles, large slugs of gas or a discrete layer of gas above the water and oil. A slip may also occur between the liquid phase and the gas phase and is more likely.
The conventional approach of industry to the characterization of multi-phase fluid flows is fluid sampling and separation. A sample of the multi-phase fluid is diverted from the flow and allowed to separate into its component phases. Once separated, measurements may be made of the individual phases using conventional single-phase flow measurement techniques and devices. This conventional approach has several drawbacks. Sampling requires the extraction of a quantity of the fluid, on either a continuous or a periodic basis, by an intrusive sampling probe. As well, homogenization of the flow may be required prior to sampling in order to obtain a representative sample of the fluid. Further, sampling and separation of the phases may be time consuming and the required equipment may be costly, bulky, complex and require ongoing maintenance. Thus, the efficiency and economics of conventional field fluid samplers and separators have not been found to be completely satisfactory.
Alternatively, Canadian Patent No. 1,134,174 issued Oct. 26, 1982 to Rhodes et al is directed at a device which measures the flow of a multi-phase fluid without sampling and separation of the phases. Rhodes describes a flow meter which is designed to measure the individual flow rates of the phases of the fluid by measuring a frictional pressure drop and an accelerational pressure drop of the fluid. The frictional pressure drop is measured across a twisted tape in a conduit carrying the flow, while the accelerational pressure drop is measured across a venturi positioned in the conduit downstream of the twisted tape. However, this device does not completely address the problems associated with the variable flow regimes, flow instability and slip in multi-phase fluid flows.
However, specific attempts have been made to address these problems as shown by Canadian Patent Application No. 2,103,254 filed by Farchi et al and published Sep. 18, 1993, U.S. Pat. No. 3,176,511 issued Apr. 6, 1965 to Widmyer, U.S. Pat. No. 4,168,624 issued Sep. 25, 1979 to Pichon, U.S. Pat. No. 4,441,361 issued Apr. 10, 1984 to Carlson et al, U.S. Pat. No. 4,856,344 issued Aug. 15, 1989 to Hunt, U.S. Pat. No. 4,974,452 issued Dec. 4, 1990 to Hunt et al and U.S. Pat. No. 5,190,103 issued Mar. 2, 1993 to Griston et al.
Farchi describes an apparatus for measuring the flow rates of the gas and liquid components of a fluid in a series flow path. Farchi states that the velocity ratio between the gas and the liquid in the series flow path is preferably maintained at a known value, such as one, by using either a first and second mixer or a positive displacement flow meter. The first and second mixers are coupled at the input and output of the volumetric flow meter. However, the specific method by which the velocity ratio is effectively maintained at one through the volumetric flow meter is not described. Further, no definition or description of the positive displacement flow meter, or the method by which it maintains the velocity ratio, is provided by Farchi.
Widmyer provides for a measuring apparatus which includes a plurality of baffle plates which form the walls of a tortuous passageway for the fluid. The fluid passes through the passageway, where it is mixed, and subsequently through a partition and into a separate fluid density measuring device. The fluid then passes through a second partition into a separate flow rate or volume measuring device. Similarly, each of Pichon, Carlson, Hunt, Hunt et al and Griston all describe devices which discuss the use of a mixer or other means, for making the fluid flow substantially uniform, which mixer is located upstream of the particular measuring devices or flow meters used in each device.
In addition, further attempts to overcome these problems are also shown by European Patent No. EP 0690292A2 by Marelli et. al. published Jan. 3, 1996, German Patent No. DE 2815651A by Yxzet published Oct. 25, 1979 and U.S. Pat. No. 4,061,313 issued Dec. 6, 1977 to Brauner et. al.
Marelli et. al. describes a device which provide a monitoring device, being a water cut monitor, located downstream of a static mixer. Thus, the mixing of the multi-phase fluid by the static mixer ceases prior to the passage of the multi-phase fluid through the monitoring device. Since the mixing of the fluid ceases upstream of the monitoring device, the fluid passing through the monitoring device has an opportunity to segregate or separate such that the fluid being monitored may not be completely homogeneous.
Yxzet also describes a stationary mixing apparatus for flowable substances. More particularly, the apparatus is comprised of a tubular housing containing packing bodies, which are preferably comprised of a plurality of spheres. The packing bodies may be densely or loosely packed as required for the particular mixing process. Further, at least one vibrator is associated with the housing for setting the packing bodies in motion. The vibrator may be internal or external to the housing and may be comprised of a jolting machine, an electromagnetic oscillator or the like. However, Yxzet does not describe or provide for the measurement of the flowable substance at any time before, during or after the passage of the flowable substance through the mixing apparatus.
Brauner et. al. is similarly directed at an apparatus for the static mixing of flowable substances. Specifically, the static mixer comprises a tubular housing having a mixing insert arranged therein, consisting of a plurality of plates having webs in intersecting planes inclined to the axis of the housing. As in Yxzet, Brauner et. al. does not describe or provide for the measurement of the flowable substance at any time before, during or after the passage of the flowable substance through the static mixing apparatus.
Each of these patents either describes the mixing of the flow of the multi-phase fluid, without describing the taking of any measurements thereof, or describes the mixing of the flow of the multi-phase fluid prior to the taking of any measurements so that the fluid may subsequently be measured by means suitable for a single-phase fluid. However, although these patents attempt to address the problems of varying flow regimes, flow instability and slip, these problems may not be completely overcome by the devices and techniques disclosed by these patents.
As stated, all of the devices and techniques disclosed by these patents attempt to achieve uniformity in the flow of the multi-phase fluid by mixing the segregated phases. Further, in the event both mixing and measuring of the fluid are described, the mixing of the segregated phases occurs prior to the taking of any measurements of it. As a result, partial resegregation or separation of the phases will occur immediately following cessation of the mixing of the phases, or once the fluid has passed through the mixer, due to the immiscibility and differences in the densities (buoyancy or gravity segregation effect) of the fluid phases.
Thus, the fluid flow subsequently measured by each of the disclosed measuring devices is not uniform or homogeneous, but rather, it is partially or completely segregated or separated into its component phases. This partial or complete segregation of the phases of the fluid flow can cause inaccuracies in the measurements being made, particularly when using measurement devices and techniques conventionally used for single-phase fluids. Conventional single-phase fluid flow measurement devices and techniques are feasible and provide relatively accurate measurements only when the multi-phase fluid flow is homogeneous or substantially uniform.
There is therefore a need in the industry for an improved method and an improved apparatus for relatively accurately characterizing the flow of a multi-phase fluid. As well, there is a need for a method and a device capable of characterizing the multi-phase fluid flow using conventional single-phase fluid flow measuring devices and techniques. Further, the device is preferably relatively compact and simple and relatively economical and easy to construct and use in the field.